The radium content in water supplies has come under intense scrutiny since the early 1980s, following the announcement by the U.S. Environmental Protection Agency of an acceptable upper limit for the radium content in drinking water of 5 picocuries per liter (5 pCi/L). It has been shown through recent surveys that this limit is exceeded in many communities, particularly those communities that obtain their drinking water from wells.
Radium-226 (Ra-226) and radium-228 (Ra-228), which are products of the uranium-238 and thorium-232 decay chains, respectively, are of particular environmental concern. Ra-226 has a half-life of 1600 years and is among the most toxic long-lived alpha emitters present in environmental samples. Thus, considerable interest has developed in improving methods for its determination. Likewise, interest has also developed in improving methods for determining levels of Ra-228 in water and other samples.
Because of the low levels of radium generally encountered in such environmental samples, radium determination generally requires a preliminary preconcentration and separation step to isolate the radium from comparatively large quantities of inactive substances and to free the radium from other radioisotopes that may interfere with subsequent counting. In a typical process, this separation and preconcentration step involves co-precipitating the radium with barium and/or lead sulfate. Following washing, the precipitate can be subjected to other treatments to further reduce the number of other radio-nuclides present or to reduce the quantity of the carrier present in the sample. Although such methods are generally effective, they can be tedious, particularly the precipitation step or steps.
In addition, radium levels in many barium reagents are not negligible, relative to the levels to which the radium is tested in the samples, which can require a preliminary purification of the reagents to reduce what is commonly referred to as "blank" levels. Moreover, when the precipitate containing radium is counted by alpha spectrometry without further treatment, the precipitation conditions must be carefully controlled to avoid degradation of the alpha spectrum.
Extraction chromatography has been used as one method for separating and preconcentrating radium from samples. In one method, a complex scheme has been developed for separating radium and various actinides from copper foils using a column of bis(2-ethylhexyl)phosphoric acid (HDEHP) sorbed on Celite.RTM. 535. A. Turler et al., Radiochim. Acta 1988, 43, 149-52. The uptake of radium by several podands and macrocyclic polyethers supported on either Amberlite.RTM. XAD resin or Kieselgel has also been recently examined. Radium uptake was observed to be both low and irreproducible using the supported macrocyclic polyethers. Similar unsatisfactory results were achieved using open-chain extractants. P. Benzi et al., Nucl. Chem. Lett. 1992, 164, 211-20.
Ion exchange techniques have also been used to separate and preconcentrate radium cations from various samples. In one procedure, three separate, successive cation exchange columns were used to isolate radium from geologic samples for subsequent mass spectrometry. A. M. Volpe et al., Anal. Chem. 1991, 63, 913-16. In another procedure, a combined anion/cation exchange method was used to isolate Ra-226 from human bone ash for subsequent electrodeposition and alpha spectrometry. M. Yamamoto et al., Radiochim Acta 1991, 55, 163-66. A single column procedure has been used to isolate radium from drinking water samples, using a strong cation exchange resin and gamma spectrometry. D. A. Clifford et al., Health Phys. 1992, 62, 413-22.
Regardless of the ion exchange procedure used, such methods, like other techniques, suffer from various limitations, and particularly, suffer from inadequate selectivity and the need for additional cumbersome sample treatment steps if the analysis involves the determination of Ra-228.
Accordingly, there continues to be a need for a simple process for selectively separating and preconcentrating Ra-226 and Ra-228 from aqueous samples. Such a process permits radium separation and preconcentration without sacrificing radium selectivity, and without the need for cumbersome sample pretreatment steps. In particular, such a method permits radium separation without requiring co-precipitating steps with various reagents that can later contaminate the sample. The disclosure that follows illustrates one such simple process for selectively separating and preconcentrating radium cations.